In an automotive vehicle it is common practice to provide power-assist means for facilitating the steering of the vehicle. To this end, a servomotor or other powered unit is coupled to the steering linkage or the steerable wheels of the vehicle and is operated in response to rotation of the steering-wheel shaft.
It is known to provide the shaft of the steering wheel of a vehicle with a device generating an output for operating the servomotor or power-assist steering unit of the vehicle.
For example, the steering-wheel shaft can be provided with means responsive to a relative rotation of one part of the shaft relative to the other, e.g. the hub carrying the steering wheel relative to the shaft connecting the latter to the steering linkage, in the form of a spring which is compressed depending upon the degree of relative rotation and hence the displacement of the steering wheel to one side or another. In this arrangement, two springs are provided which are compressed alternatively depending upon the sense of rotation of the steering wheel. Upon compression of either of these springs, a respective contact is closed to generate an electric signal and operate a motor for controlling the power-steering system. This arrangement, however, has the disadvantage that it only is capable of providing an ON-OFF signal representing the side to which the steering wheel is rotated and hence the direction to which the steerable wheels of the vehicle are to be turned. A system of this type can be found, for example, in German Pat. No. 613,383.
In another known system for operating the power-steering mechanism of a vehicle, the steering wheel is connected via the steering shaft by four tangentially oriented carbon-compression resistors and these resistors are connected in a bridge circuit. The force between the steering wheel and the steerable wheels is thus transmitted to the steering shaft via these carbon-compression resistors and the resistors generate a continuous signal which varies as a function of the degree of compression. In this arrangement, an additional elastic connection between the relatively rotatable parts of the system connecting the steering wheel to the steering mechanism is not provided and the resistors must take up the full steering force to their detriment.
It is also known to provide means for signaling the torque in a shaft by providing the surface of the shaft with helical or spirally arranged strain-gauge members which can also be connected in a bridge circuit. The resistor system is described in French Pat. No. 1,185,576.
All of the systems discussed above suffer from excessive complexity, unreliability and failure after only a brief useful life.